Forming Convex and Concave Surfaces

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If this is all a logical progression, then the next step is to machine rises and depressions, using the same basi c technique. If the tracks arc cut to a concave or convex curve, the router carriage will rise and fall (or fall and rise, as the case may be) as it is advanced along them. The bit will penetrate deeper at one point than at another.

If you like cabinets or other furniture pieces with curved surfaces, this is a great way to produce those surfaces.

The surfacing platform can be adapted to this work easily. Just change the tracks. The stock carriage should work fine. (You don't have to keep the fences when the job's done. Unless you arc going to use them again and again, just toss them.)

Work out your curve, laying it out directly on a piece of the track stock or laying it out on a template. A good way to find a fair curve requires a relatively thin strip of wood, plywood, acrylic, plastic laminate, or even metal as a guide. Flex it so it

The only real difference between flattening a board with the surfacing platform and scooping out a hollow with it is the contour of the tracks. With these curved tracks, it's easy to hollow a plank, something you can't do on a jointer or with a planer.

bows. Have a helper trace along the strip to record the curve. You can mark starting and ending spots, as well as the deepest (or highest) pan of the curve, anil develop an even curve that connects them. (Check through the previous chapter, "Routing Curves and Circles." for ideas on using your router to create the arcs you need.)

However you do it. lay out and then cut the curve for the tracks. Make the curve as smooth and even as you can. If you can get one good track, use a your router and a flush-trimming bit to produce a duplicate.

Ijtying out a smooth, even curve calls for a flexible strip of thin wood, metal, or, as here, acry lic, and a couple of hand screws. Stand the strip on edge and clamp one end to the workpiece. Hex the free end until you have a curve that appeals to you. Clamp the free end, then trace the strip's shape onto the work.

Drill the mounting holes, install the T-nuts. and mount the tracks on your surfacing platfonu.

To get the curve aligned properly on the workpiece. you have to position it carefully between the tracks. Scribe centerlincs across both the tracks and the workpiece. When you fit the work between the tracks, line up its centerline with those on the tracks. Fit scraps against the ends of the work and clamp them to the platform. This will prevent the work from shifting out of alignment. Then use the wedges to secure the work-so it doesn't shift from side to side

Set the carriage in place and drop the router into it. If you are using a plunge router—it will make it easier to get to the finished depth if you do—slide the carriage to position the bit at the beginning of the cut. Lower the bit until it just touches the wood, then set the depth stop so that this is the deepest the bit will be able to cut.

To begin routing, raise the bitso you'll be taking olf between '/«inch and '/« inch at the point of deepest penetration. Stan with the carriage at one end of the track and the router positioned at one side. Slide the car-





Scoop out ti stool scat on your surfacing platform! With scat blank mounted on the turntable and the router pinned in the middle of the carriage, simply feed it back and forth on the curved tracks. To keep the turntable from moving when you make a pass, stick a strip of scrap between the track and the turntable. Just use a strip that's long enough to keep your hand clear of the router.

Customize your surfacing platform so you can use it to hollow seats for stools. All it takes is a lazy-Susan bearing and a couple of plywood squares to make a swivel base.

To use the swivel, you mount a blank on it, then position it between die concave fences. Chuck the appropriate bit in the router, set the cutting depth, then lock the router in the center of its carriage. As you make a pass with the router, hold the blank. (The swivel spins easily, and you'll quickly discover that the bit can make it really whirl.) When the bit is free of the work, tum the blank ever so slightly. Make another pass. Turn the blank again, and make another pass. Just keep at it until the entire blank is hollowed.

Making the swivel is easy. You need a 6-inch lazy-Susan bearing. The base is an 11-inch-square piece of /«-inch plywood. Scribe center-lines on it, dividing it into quarters, then scribe diagonals. Set the bearing on it and mark where you will drive screws that will mount one of the bearing's flanges to the base; these points should be on the diagonals. On the centeriines, mark the centers of the access holes. Cut the access holes, including one at the center of the base. Now screw the bearing to the base.

Lay out and cut the turntable next. Its diameter should equal the diagonal dimension of the bearing. Drill a pilot hole at the turntable's center. Then mount the turntable on the bearing. Be sure it is centered.

To mount a blank on the turntable. drive a screw through the centerpoint of the turntable into the blank's centerpoint. One screw is all it takes to secure the work.


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nage to the other end of the tracks, shift the router slightly, and return. Shift the router and make another pss. Work methodically in this fash-en until you have completed a first pass over the entire workpiece. Plunge a little deeper and repeat the process.

Make as many additional passes its necessary to achieve the curve you've laid out. Make the final pass a skimming cut to leave the surface as smooth as you can get it with the router. Scraping and sanding will then remove any remaining tracks, swirl marks, or uncvcnness.


rThe early woodworker dealt with wood's instability using frame-and-pancl construction. The rails and stiles forming the frame are relatively narrow, and ihe dimensional changes in it that accompany humidity changes are correspondingly modest. While the panel is much wider than the stiles, it's set into a groove or rabbet in such a way that dimensionat changes are "absorbed" without damaging the structure, even cosmetically. So the frame contributes dimensional stability, and the panel (usually) contributes good looks.

The most commonplacc examples of frame-and-pancl construction arc doors and windows. It's used in elaborate architectural paneling and in custom cabinetry and furniture. In these uses, frame-and-pancl construction is styled to be attractive as well as functional. But inside casework, it can be functional, and appcarance is of little consequence.

The router can be a pivotal tool in profiling and joining the frame members atid in shaping the panels. In a well-equipped production shop, the shaper is the tool used. But the router can do the job. regardless of the joinery selected.

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  • fre-qalsi
    How to route a concave surface?
    2 years ago

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